I think, for folks like Rory (and me, the way I did it), the guitar parts while you're singing are very easy/automated. You are just strumming some tastful accompaniment - the less you put in, the easier, and probably better it gets. And then you learn a few tricks to slot in between lines - make it seamless enough and everyone thinks you're playing complicated guff all the way through...
But, like you say, the bassist is part of the rhythm section and is usually playing a single note line that MUST NOT HESITATE... with a lot of music, the guitarist can get away with sloppy rhythm on most of the chords, if the bassist waivers in most music, the whole band starts sounding tentative (at best).
However, most singer-bassists (eg Sting) do have very simple bass parts under the vocals - I partly want to listen to Robin Trower tonight to check James Dewar's bass playing out. Even Jack Bruce's seemingly interesting bass parts seem to have been built for easy sing-a-long. I can't play Phil Lynnot parts and sing his lines, though - something to do with how the rhythms work. It was obviously easy for him, watching film of him, but I can't keep it going...
And then Geddy Lee - he's like two different people simultaneously. Early Rush stuff, the bass is complicated, but his vocal is following it. Later Rush stuff, how the f@ck does he do it!!! :lol: (and I'm talking about live footage here).
To anyone trying to figure out how to sing and play - I believe the trick is to concentrate on the singing first - that's what the general punter is listening to. You have to sing with a certain amount of conviction, if your mind is worrying about playing the guitar right, your voice sounds like an afterthought and the audience doesn't buy in to the performance as much as they could. So while you're working the vocal up, you have to look at the guitar part and cut out the clever sh1t you thought you were going to play... you have to be able to play that part blind (and possibly without being able to "hear" it), so don't be too ambitious. Figure out the minimum you can guarantee, learn that and concentrate on the vocal during the gig. Later, with experience and bravado you can start feeling/behaving like Jimi Hendrix, where the vocal and guitar are part of one thing coming from one person - but you can't do that without a lot of self-confidence, and you won't build any of that up by being over-ambitious early on and messing it up in front of people!
